Caustics are light effects that occur if light is reflected or refracted by specular reflective or refractive
surfaces. Imagine a glass of water standing on a table. If sunlight falls onto the glass you will see spots of light
on the table. Some of the spots are caused by light being reflected by the glass while some of them are caused by
light being refracted by the water in the glass.

Since it is a very difficult and time-consuming process to actually calculate those effects (though it is not
impossible, see the sections "Photons") POV-Ray uses a quite simple method to simulate
caustics caused by refraction. The method calculates the angle between the incoming light ray and the surface normal.
Where they are nearly parallel it makes the shadow brighter. Where the angle is greater, the effect is diminished.
Unlike real-world caustics, the effect does not vary based on distance. This caustic effect is limited to areas that
are shaded by the transparent object. You will get no caustic effects from reflective surfaces nor in parts that are
not shaded by the object.